Influenza, yearly, causes approximately 36,000 deaths in the United States. Many of the deaths are secondary to pneumonia or cardiac complications and are much more common in the older than in the younger populations. In the past several years there has been increasing concern over rising rates of P&l deaths in the elderly despite higher vaccination rates than ever before. This concern became a public debate in 2005 with the article by Simonsen that asserted that observational studies of vaccination overestimated the benefits of vaccination. Due to the disproportionate morbidity and mortality of influenza for people >65 years of age, public health officials in the United States have recommended routine yearly influenza vaccination to individuals = 65 years of age and more recently added all adults over 50 years of age to the recommendation. These decisions were intended to reduce the morbidity and mortality of influenza but were not based on randomized clinical studies of vaccine efficacy in older individuals. Only three randomized clinical trials (RCTs) testing the efficacy of trivalent inactivated influenza vaccines have been conducted in elderly adults and only one used laboratory-confirmed influenza as an endpoint. Even though vaccine efficacy (VE) was 56%, this study was not adequately powered to examine VE in adults >70 years of age and only looked at healthy elderly. Hence many questions still exist about the effectiveness of influenza vaccination in older adults. This application is a sub-study of a larger surveillance program conducted by Dr. Talbot's mentor, Dr. Kathryn Edwards. This surveillance study will prospectively test hospitalized patients for influenza with culture, antigen and PCR. With this information the following aims will be addressed: 1. To estimate the effectiveness of trivalent inactivated influenza vaccine (TIV) against laboratory-confirmed influenza-associated hospitalizations in individuals aged >50 years of age. 2. To determine the sensitivity and specificity of both the CDC-defined influenza-like illness (I LI) and rapid influenza antigen detection using the gold standard diagnosis of either a positive influenza culture or a positive reverse transcriptase - polymerase chain reaction (RT-PCR) for influenza in individuals aged >50 years of age hospitalized for acute respiratory illness. 3. To describe the morbidity associated with hospitalized episodes of influenza in individuals a 50 years of age, using prospectively collected population-based data.